Equivalent circuit definition and calendar aging analysis of commercial Li(NiMnCo)O2/graphite pouch cells

Aging-induced degradation of commercial Li-ion pouch cells with lithium nickel-manganese‑cobalt-oxide-based cathodes and graphite anodes is studied at various operating conditions (temperature and voltage) by galvanostatic measurements and in situ electrochemical impedance spectroscopy (EIS). A detailed equivalent electrical circuit model, capable to fit the measured EIS spectra, is developed and validated. The work also confirms the capacity fade dependence of the calendar-aged cells on stress factors such as temperature and state-of-charge. The retained capacity is found to decrease linearly with t0.5. However, it stands out that the degradation registered for cells held at 95% state-of-charge is lower than that for those at 70% and for the cases when temperatures are between 25 °C and 37.5 °C, which is a not very common singularity. A high performing calendar aging model is introduced accordingly. Both the electrical circuit model defined and the accurate calendar aging model developed provide useful tools for battery management systems in order to monitor and control both the state-of-health and the state-of-charge of these commercial cells.